The present invention relates to an apparatus and method for acoustically guiding, positioning, and monitoring a tube or catheter within a body. More particularly, the present invention relates to an apparatus and method to guide the placement a tube in a body conduit or cavity, to monitor the position of the tube, and to insure the patency of the tube in the body using a noninvasive acoustic reflectance technique.
The specific example of placement of an endotracheal tube (hereinafter "ETT") inserted into the airways of a respiratory system is disclosed. However, it is understood that the acoustical guidance apparatus and method of the present invention may be used in connection with guiding other types of tubes, catheters, or similar devices into various body conduits or cavities.
For the case of ETT placement, the structure of the human airways are extremely complex. The ends of the trachea are characterized by the larynx (which contains the vocal folds) cranially and a first bifurcation, known as the carina, caudally. The adult trachea is approximately 1.4 to 1.6 cm in diameter and 9 to 15 cm long. The newborn trachea averages about 0.5 cm in diameter and 4 cm in length. The airways that are formed by the carina are the right primary bronchus and the left primary bronchus. The right primary bronchus is shorter, wider, and more vertical than the left primary bronchus. For this reason a majority of ETT insertions past the carina tend to follow the right primary bronchus. Continuing farther down the airways, the bronchial tubes branch into smaller and smaller tubes. They finally terminate into alveoli, small airfilled sacs where the oxygen-carbon dioxide gas exchange takes place. The lungs consist of approximately 10.sup.7 airways with 35 branching generations terminated into nearly 300 million alveoli.
An ETT is inserted through the mouth and into the trachea of a patient for several reasons: (1) to establish and maintain an open airway; (2) to permit positive pressure ventilation which cannot be done effectively by mask for more than brief periods; (3) to seal off the digestive tract from the trachea thereby preventing inspiration of forced air into the stomach; and (4) as an anesthesia delivery system. Endotracheal intubation may be complicated by inadvertent insertion of the ETT into the esophagus, or past the carina into one of the right primary bronchus or the left primary bronchus. Improper placement of the ETT into the esophagus is most evident in the emergency room setting characterized by high stress and limited time. Insertion of the ETT past the carina will result in ventilation of only the right or left lung. Also, postplacement movement of the distal ETT tip either past the carina or above the vocal folds due to patient or ventilator tube movement, or mucous blockage of the ETT lumen, can occur over time. In all of these scenarios, the patient is ineffectively ventilated which may result in severe medical complications.
In an attempt to avoid these possible complications, techniques have been developed to aid clinicians in the determination of the location of an ETT. The general guidelines for an ideal method are as follows: (1) the test should work for difficult intubations; (2) positive tests which indicate a proper ETT tip location must be unequivocal; (3) esophageal intubation must always be detected; and (4) clinicians must understand the test. The known techniques for clinical evaluation of ETT location include stethoscopic evaluation of airway, breath, and epigastric sounds, respiratory system compliance measurements, detection of asymmetrical chest excursion, chest compression techniques, palpation of the ETT cuff over the extrathoracic trachea, electromagnetic detection devices, ultrasonic techniques, optical techniques, carbon-dioxide measurements, suctioning devices, and chest x-rays.
Due to various shortcomings, only chest x-rays and carbon-dioxide measurements are used in a widespread clinical manner. The chest x-ray technique suffers from lengthy assessment time, significant cost, and radiation exposure if multiple measurements need to be taken. The carbon-dioxide technique cannot be used to determine the exact position or patency of the ETT in the respiratory tract, or to directly detect bronchial intubation. In other techniques, specialized apparatus and skilled clinicians are required, and accuracy may depend more on the ability and experience of the clinician than the actual technique. In addition, the clinical need remains for an instrument that can continuously monitor ETT position and patency after placement.
Audible sound has been used to determine the location of a suctioning catheter within the airways. See, for example, United Kingdom Patent Document No. 2,068,735 to Kubota. The sound is introduced into the proximal end of the catheter, exits the distal end, and is detected by a stethoscope on the chest wall. The chest wall location of the strongest sound signal is used to determine catheter location. This method does not include a microphone to monitor acoustic reflections from the tube or catheter and has not been used to assure proper positioning of an ETT within the trachea.
Techniques have also been developed that employ audible sound reflections to determine physical characteristics of passageways in living subjects such as the airway from measurements-made at the mouth. See, for example, U.S. Pat. No. 4,326,416 to Fredberg. These techniques have not been used in conjunction with a moveable tube or catheter such as an ETT to guide placement, determine position, or insure patency within the body.
The acoustical properties of the airways of a respiratory system change dramatically over the audible frequency range. At very low frequencies, the large airway walls are yielding and significant wall motion occurs in response to intra-airway sound. In this frequency range, the airways cannot be represented accurately as rigid conduits and their overall response to sonic pulses is predictably complex. At very high audible frequencies, the large airway walls are effectively more rigid due to their inherent mass. However, one-dimensional sound propagation down each airway segment cannot be insured as the sonic wavelengths approach in size the diameter of the segment and effects of airway branching are thought to increase in importance. There appears to be a finite range of frequencies between roughly 500 and 6,000 Hz where the large airways behave as nearly rigid conduits and the acoustical effects of the individual branching segments are not dominant. It is over this limited frequency range where the complicated branching network can be approximately represented as a flanged "horn" and where its composite acoustical properties reflect the total cross-sectional area of the airways.
The present invention advantageously exploits the acoustical properties of the airways and provides a noninvasive instrument to monitor ETT position. Design criteria for the present invention included that the instrument be: 1) able to distinguish between esophageal, tracheal, and bronchial intubations; 2) sensitive to small movements of the tube; 3) able to continuously monitor ETT position over time; 4) noninvasive.
In the present invention, an audible sound pulse is introduced into a wave guide and is recorded as it passes by a microphone located in the wave guide wall. The sonic pulse then enters the connected proximal end of the ETT, propagates down the ETT, and is emitted into the airways. Acoustic reflections occur within the airways and are recorded by the same microphone as they propagate back up the wave guide. A well-defined inverted reflection arises from the point where the total cross-sectional area of the airways increases rapidly, and the difference in timing between the detection of the incident pulse and this reflection is used to determine ETT position or movement. In addition, the amplitude and polarity of a reflection arising from the abrupt change in area between the ETT tip and the airway in which the ETT is placed is used to estimate the cross-sectional area immediately following the ETT distal tip. This information allows discrimination between tracheal and inadvertent bronchial intubation and can be used to insure an adequate fit between ETT and trachea. In the case of an erroneously placed ETT into the esophagus, the well-defined inverted reflection is not observed and the estimated diameter immediately following the ETT tip is erratic and occasionally less than that of the ETT. Mucous fluid deposition within the ETT is detected and quantified by the resulting increased sonic reflections. The instrument has proven extremely reliable in multiple intubation procedures in eight canines and thus noninvasively, reliably, and inexpensively monitors ETT position and patency in a continuous manner.
The characteristics of the reflected pulses that were measured from the airways were similar to that predicted by the aforementioned simple airway model. Four of its key features provide strong support that the airways behave similar to a tube with a flange at its end: the airway reflection's 1) timing corresponded to a boundary roughly 15 to 20 cm below the vocal folds, dependent on canine size, 2) amplitude was inverted as compared to the incident wave indicating an increase in total cross-sectional area A, 3) duration was short (0.7 ms) and 4) energy was a significant fraction (on the average 70%) of that of the incident airway pulse. The last two features indicate that the change in A is large and occurs rapidly from a spatial perspective, as was predicted from the model.
The determinant of the reflection resulting from the change in A between the ETT tip and the airways was confirmed by occluding the ETT on the bench top and observing the reflection polarity change as compared to the unoccluded case. This reflection is extremely useful for three important reasons. Firstly, it provides an index of the appropriateness of fit between the diameter of the ETT and the trachea, and potentially allows the user to optimally match the diameter to the patient via equation (6). ETTs having three different inner diameters of 0.9, 1.0, and 1.1 cm were employed in this study and the procedure of matching ETT size to the animal worked quite well. Secondly, in the case of an erroneous esophageal intubation, it can provide a clear-cut mechanism to discriminate these from proper placement in the trachea through detecting a non-inverted reflection from this boundary owing to a smaller esophageal A as compared to that of the ETT. Since the location of the ETT tip relative to the microphone does not change, it is a simple analytical procedure to assess if this reflection is not inverted and incorporate this information into the guidance system. Lastly, it can be employed to detect movement of the ETT past the carina and into a bronchus, as determined by the decrease in estimated airway cross-sectional A.sub.tube.
The instrument meets all of the stated design criteria. It is completely noninvasive since it employs audible sound at maximum sound pressure levels of &lt;120 dB SPL (re 0.0002 .mu.bar) which are comparable to those found within the lower vocal tract during normal speech, and are barely audible when connected to the intubated ETT. It is relatively simple in terms of components, requiring a wave guide, speaker, microphone, valve, and signal conditioning/processing hardware. Since measurements can be made either very rapidly or slowly, it can be used to guide placement or monitor changes in ETT position over long periods of time in patients who require extended ventilatory assistance. For these latter cases, ETT patency information is also available. The absolute accuracy of the device to &lt;0.7 cm over the insertion length of an entire intubation procedure is adequate to be helpful in clinical situations in adults as well as infants. Also, the differences between the acoustic signals measured for the ETT in the trachea, esophagus, and bronchus provide useful information to guide the placement of the ETT into a proper location for ventilation. Once the ETT tip is placed through the vocal folds and the airway reflection is detected, the ETT can be advanced a preset distance below the vocal folds as guided by the instrument. A "safety zone" can then be defined, with the system indicating through an alarm if the ETT has inadvertently moved outside the zone and that repositioning is required.
According to one aspect of the invention, an apparatus is provided for acoustically guiding a distal end of a tube within a body. The apparatus includes means coupled to the tube for generating an incident sound pulse in the tube which propagates into the body, means for detecting sound pulses in the tube resulting from the incident sound pulse and from reflected sound pulses from within the body, and means for processing the detected sound pulses to guide insertion of the distal end of the tube within the body.
In the illustrated embodiment, the processing means includes means for providing an indication of the position of the distal end of the tube within the body, and means for displaying information generated by the processing means. The apparatus also includes means for monitoring the position of the distal end of the tube within the body. The monitoring means generates a warning signal if the distal end of the tube moves beyond a preset safety zone.
Also in the illustrated embodiment, the processing means includes means for estimating dimensions of the body adjacent the distal end of the tube. The apparatus includes means for generating a warning signal if the dimensions estimated by the estimating means are smaller than dimensions of the distal end of the tube.
The processing means also includes means for determining if the tube is obstructed. The apparatus further includes means for generating a warning signal if the tube is obstructed by more than a predetermined percentage.
Illustratively, the apparatus includes a wave guide having a first end coupled to a proximal end of the tube and a second end, a mechanical ventilator, and a valve movable from a first position to provide communication between the wave guide and the proximal end of the tube to a second position to provide communication between the mechanical ventilator and the proximal end of the tube. The illustrated apparatus further includes comprising an absorptive material coupled to the second end of the wave guide for substantially absorbing sound pulses moving toward the second end of the wave guide.
It is understood that in certain other applications the wave guide may be modified or eliminated. For certain applications, sound pulse generators and detectors may be coupled directly to the tube, either externally or internally.
According to another aspect of the present invention, a method is provided for acoustically guiding a distal end of a tube within a body. The method includes the steps of generating an incident sound pulse in the tube which propagates into the body, detecting sound pulses resulting from the incident sound pulse and from reflected sound pulses from within the body, and processing the detected sound pulses to guide insertion of the distal end of the tube within the body.
In the illustrated method, the processing step includes the steps of providing an indication of the position of the distal end of the tube within the body conduit, estimating dimensions of the body conduit adjacent the distal end of the tube, and determining if the tube is obstructed. The method further includes the steps of monitoring the position of the distal end of the tube within the body, generating a warning signal if the distal end of the tube moves beyond a preset zone, and displaying information generated during the processing step. The method still further includes the steps of generating a warning signal if the dimensions estimated by the estimating means are smaller than dimensions of the distal end of the tube, and generating a warning signal if the tube is obstructed more than a predetermined percentage.
According to a further aspect of the present invention, a method is provided for acoustically guiding a distal end of a tube within a body conduit containing a medium using propagation of sound pulses. The method includes the steps of determining acoustic properties of the medium in the body conduit through which the sound pulses must propagate, determining acoustic properties of a wall of the body conduit, and locating identifiable boundaries within the body conduit which are capable of providing identifiable sound reflections. The method also includes the steps of optimizing characteristics of the sound pulse to provide detectable sound reflections from within the body conduit, generating an incident sound pulse having optimized characteristics in the tube so that the incident sound pulse propagates down the tube and into the body conduit, detecting sound pulses in the tube resulting from the incident sound pulse and from reflected sound pulses from within the body conduit, and processing the detected sound pulses to guide insertion of the distal end of the tube within the body conduit.
Additional objects, features, and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of a preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.